="VegaBabyThere were also some tets done which showed that low setting of the output control can lead to that. I think if one runs the output quite hot it's a lot less...
That depends entirely upon the specific pot in your AFX. In some cases, this will be true, but in other cases, it will be the opposite. You have to consider both the max value of each unit in the gang plus the taper of each unit in the gang. If you were unlucky enough to have a 0.8M (-20%) on the right and a 1.2M (+20%) on the left and the taper exactly perfect (which NEVER happens), you would have more disparity with the output dimed than you would at lower settings. But the taper of a carbon pot generally has more variance than the min/max values, so you have to include it in the equation. The taper of each unit in the example above could make the variance more or less at any given location on the dial--especially since we are talking about the notoriously inaccurate pseudo-logarithmic taper used in most commercial pots.
The "tets" you reference are purely anecdotal as the results will vary from one pot production lot to the next. In a few months, the same "tets" might yield the exact opposite results.
This is why most high-end stereo mic pres use rotary switches with 1% resistors to set output levels. This approach is less versatile in terms of continuous adjustment, but much more accurate in terms of level control from right to left.